The term “monolithic” as used herein refers to a single piece integrated circuit or module. In connection with the use and operation of integrated circuits attention must be paid to protecting the circuit arrangement, more specifically the terminals of the circuit or its connection pins or connector tabs against excess or transient voltages. Such voltages can interfere with the normal operation of the circuit and may even damage the integrated circuit components. Transient voltages can be generated, for example by the capacitive and/or inductive coupling between conductors that are arranged in common on the circuit board. It has been noted that, for example in connection with so-called CAN-bus systems, positive as well as negative transient voltages may occur with amplitudes of up to ±45V. Such bus conductors are installed in motor vehicles in the form of a cable harness. The transient voltage is measured relative to the substrate or relative to the reference voltage of the integrated circuit which represents the lowest potential of the integrated circuit. Another source for transient voltages are so-called electrostatic discharges (ESD) which can generate low energy, yet high amplitude voltage peaks.
Conventional circuit arrangements for the protection against transient voltages of integrated circuit limit these transient voltages which are positive relative to a reference voltage, to an adjustable limit value. When a transient voltage at a terminal or contact pin is generated and exceeds the adjustable limit value, circuit elements become conductive for thus limiting the voltage occurring at the particular terminal. This type of excess voltage limitation is also known as a so-called clamp voltage arrangement or simply clamp mechanism.
Normally, the circuit arrangements or circuit components for the protection against excess voltages are monolithically integrated into the integrated circuit to be protected because discrete or external not integrated circuit components make the design of the integrated circuits more difficult and do not assure a respective safety in operation.
In case voltages that are negative relative to a substrate reference voltage or potential, occur at a terminal, it is possible that parasitic diodes such as box diodes, bulk diodes or substrate diodes become conductive, whereby an interference with the proper operation may occur or the integrated circuit may be damaged. Such negative transient voltages are conventionally clamped to a level which is lower than the substrate reference potential. This lower voltage level corresponds to the diode conducting voltage of a protective diode that functions as a voltage limiting element. Stated differently, the protective diode becomes conductive when the voltage present at the respective terminal in its absolute value exceeds the diode conducting voltage. Thus, the voltage limits at which a clamping mechanism becomes effective are different for positive and negative excess or transient voltages in conventional protective circuit arrangements.
The voltage limit differences can lead to problems, particularly in a differential data transmission. Two data conductors are required in a differential data transmission, whereby the voltage difference between two data conductors are used for encoding the character or signal to be transmitted. Each of two data conductors is respectively connected to a corresponding terminal of a driver circuit stage, whereby the two terminals form a terminal pair. If a logic “1” is being transmitted, the same potential is present at both terminals. If a logic “0” is transmitted, one of the terminals assumes a high potential in the range of a supply voltage and the other terminal assumes a low potential in the range of the reference voltage or potential. This type of data transmission is substantially non-sensitive relative to common mode interferences on both conductors provided the terminals of the driver stage permit the presence of positive and negative voltage amplitudes without distorting these positive and negative voltage amplitudes, particularly by a clamp mechanism.